Source

File: wp-includes/class-wp-query.php

public function is_category( $category = '' ) {
		if ( ! $this->is_category ) {
			return false;
		}

		if ( empty( $category ) ) {
			return true;
		}

		$cat_obj = $this->get_queried_object();

		$category = array_map( 'strval', (array) $category );

		if ( in_array( (string) $cat_obj->term_id, $category, true ) ) {
			return true;
		} elseif ( in_array( $cat_obj->name, $category, true ) ) {
			return true;
		} elseif ( in_array( $cat_obj->slug, $category, true ) ) {
			return true;
		}

		return false;
	}